It is known that during such treatments, it is necessary to dewater the biological sludge from the reactor, this sludge previously undergoing a chemical conditioning to ensure its flocculation. Polymers and, more generally, polyelectrolytes are used for this conditioning, particularly in order to obtain bulky, well-differentiated flocs, in a clarified interstitial water. The liquid effluent from the dewatering operation is recycled to the head of the membrane bioreactor.
The return of this effluent to the head of the membrane bioreactor incurs a major risk due to the fact that this effluent, produced by the sludge dewatering operation, contains relatively large residual quantities of polyelectrolytes that are liable to cause severe, indeed irreversible, clogging of the bioreactor membranes.
During the treatment of waste water using a membrane bioreactor, the daily recycle rate at the head, that is of the liquid effluent from the dewatering operations, routinely represents 1 to 5% of the daily urban waste water feed rate and sometimes more than 10% of the daily rate when industrial waste water is treated.
When the sludge dewatering treatment and the recycling of the liquid effluent from this dewatering to the head of the bioreactor are carried out in batch mode, which is often the case, the proportion between the effluent from the dewatering and the waste water feed to the bioreactor may occasionally be much higher, thereby further aggravating the risk of clogging the membranes of the water treatment system, that is of the membrane bioreactor.
To overcome this drawback, two alternatives are proposed today by a person skilled in the art:    1) Avoid the risk of clogging. In this case, the recycling of the liquid effluent to the head is prohibited. The treatment of the sludge from the membrane bioreactor is then transferred to a nearby conventional station. In fact, it is not systematically possible to install such a sludge treatment station near the waste water treatment station and, in any case, this solution implies the transport of volumes of sludge that may be difficult to accept if the station is large.    2) Control the risk:            a) by minimizing the quantity (that is the batching) of polyelectrolytes used for conditioning the sludge subjected to the dewatering treatment, by making sure to reintroduce the recycled liquid effluent from the sludge dewatering treatment to the head of the bioreactor, at the point furthest from the bioreactor membranes, and to spread these recyclings to the head over time, in order to guarantee the greatest possible dilution with the waste water fed to the membrane bioreactor. This may in particular result in the need to provide a buffer tank temporarily storing the effluent before its recycling to the head.                    This alternative can help to manage the risk of clogging the bioreactor membranes, but it does not eliminate the risk of an accidental overbatching of the polyelectrolytes during the sludge treatment. It is especially difficult to control such a risk because the quantitative analysis of the residual polyelectrolytes present in the head recyclings is technically complex, indeed impossible today.                        b) by subjecting the effluents from the sludge dewatering treatment to a pretreatment in order to destroy the residual quantities of polyelectrolytes. However, this solution presents the drawback of being very costly because the removal of a few milligrams per liter of residual quantities of polyelectrolytes usually involves the at least partial removal of the pollution from the effluent produced by sludge dewatering. Thus, for example, an oxidation treatment by ozone, of the residual quantities of polyelectrolytes, implies a very large and uneconomic batching due to the ozone demand of the effluent (oxidation of organic matter). More generally, the oxidizing treatments carried out on such effluents can also give rise to oxidation by-products that are difficult to remove by the water system if the latter is not designed to treat this type of induced pollution.        
Finally, in the absence of a genuine “physical barrier”, the pretreatments proposed for the time being do not guarantee the total elimination of the risk of clogging the membranes of the water treatment system.
On the assumption that these membranes are seriously clogged by the polyelectrolyte, a person skilled in the art can apply chemical washing procedures to restore the performance of the membranes. However, the effectiveness of these procedures is haphazard and the chemicals they employ are aggressive to the membranes, jeopardizing their service life. Furthermore, these procedures involve costly large-scale maintenance and the immobilization of a portion of the membrane area that is then no longer available for waste water filtration. This drawback results in the need to oversize the membrane filtration portion.